In recent years, development and practical utilization are being promoted for a solar heat collector, a sunlight collecting system, and a solar energy utilization system which uses thermal energy of collected sunlight, as an apparatus or a system which uses thermal energy obtained from collected sunlight as a heat source for various kinds of systems and processes such as a power generation system and a chemical reaction process. For instance, there was a demonstration test of a light collecting system (in a tower method) which uses a lot of reflectors (referred to as “heliostat”, hereafter) placed on the ground to collect reflected sunlight to a heat collector placed on a top of a tower with a height of approximately 100 m, in the Solar II project conducted by the U.S. Department of Energy. In the light collecting system, a plurality of pipes are arranged in parallel in positions where the sunlight is collected by the heat collector. Thus, molten salt which circulates through the pipes is indirectly heated by thermal energy of the collected sunlight. Then, it was examined that steam generated by heat of the heated molten salt is supplied to a steam turbine to generate electric power (refer to Non-Patent Document 1).
However, in an apparatus used in the Solar II project, a light receiving surface to receive sunlight, that is, an outer peripheral side of the pipe is exposed to the outside air. Thus, the light receiving surface is deprived of a great quantity of heat by wind, and sunlight reflected on the light receiving surface and thermal radiation from the light receiving surface disperse into the surroundings. Accordingly, a ratio of thermal energy actually used to heat the molten salt with respect to energy of incident sunlight becomes small. Therefore, an efficiency of sunlight utilization is limited.
Moreover, in Non-Patent Documentation 2, as shown in FIG. 40, a sunlight collecting system (in a beam-down method) is disclosed, in which a light collecting reflector 62 is placed in a high place a little downward from a light collecting point F of light reflected by a plurality of heliostats 61 placed on the ground. Thus, sunlight is reflected downward to the ground by the light collecting reflector 62 to be collected to the heat collector 63 near the ground. Moreover, as a heat collector used in the sunlight collecting system, an apparatus is disclosed in which molten salt poured in a circular flow path formed in a space between double nested heat collecting containers in truncated cone shapes. Then, the sunlight irradiates an inside of the heat collecting container to indirectly heat the molten salt.
However, in Non-Patent Documentation 2, there is no discussion about optimizing the shape of the heat collector. In addition, in the heat collector, the molten salt flows slowly inside the circular flow path so as to have a longer response time to control a temperature corresponding to changes in a temperature of the molten salt in an outlet of the heat collecting container when a quantity of solar radiation changes. Therefore, it is difficult to finely control the temperature by flow control. In addition, it is difficult to produce a large-scale apparatus in such a complicated shape as double truncated cones so that it is difficult to achieve practical and commercial utilization.
In addition, in the beam-down sunlight collecting system shown in FIG. 40, an efficiency of sunlight utilization is further improved. However, in a light collecting system in which heliostats are installed within an area of 100 m in radius, a radius of the light collecting reflector 62 is several tens meters or more, and installation height is approximately 100 m. In this case, the reflector is subject to a high wind pressure. Therefore, a pulse of the wind displaces the position of the reflector or transforms the reflector itself so as to cause an accuracy of light collection to decrease. Moreover, a structure to support the reflector needs to be firmly constructed to stand against the strong wind during stormy weather, so as to cause a construction cost to increase. To solve such a problem, countermeasures are devised, in which the reflector is divided into small segments and the segments are placed in such a way that there is a space between adjacent segments or segments are thinned out at a predetermined rate so that gaps are made through which the wind passes. However, in these methods, it is impossible to use the sunlight which is collected by the heliostats and reaches in portions of the spaces between the reflectors. As a result, a light collection efficiency decreases. In addition, 1) the sunlight is reflected on the reflector so that a light path length becomes long; accordingly, the focus becomes relatively wider on the light receiving surface (the focal plane) so as to cause the heat collector 63 to be large. Moreover, 2) due to shortage of light collection caused by above-mentioned 1), when the molten salt collects heat in the heat collector 63, a temperature of the molten salt rises insufficiently. 3) When the heat collector 63 is placed near the ground, the light collecting reflector 62 is formed along a hyperboloid of revolution so that incident light heat fluxes are more densely distributed near the focus on the light receiving surface. This feature is not preferable to be applied to a system such as a reforming reactor or the like, where it is more advantageous that the incident heat fluxes are equal. To equalize density of the heat fluxes is also a problem to be solved by the beam-down light collection method.
Next, in the sunlight collecting systems disclosed in Non-Patent Documents 1 and 2, the heat collector or the light collecting reflector is provided at a light collecting point formed by a plurality of heliostats.
However, these sunlight collecting systems are systems still in experimental phases, in which a quantity of collected light is comparatively small. Therefore, to construct a large-scale light collecting system which can collect a sufficient quantity of light to be used in a commercial scale, the systems cannot cope with the problems which occur as the systems are enlarged. For instance, height where the heat collector or the reflector is installed is actually limited caused by influence of the wind pressure to which the heat collector or the reflector is subject.
Moreover, in Non-Patent Document 3, a light collecting system in which a plurality of towers are placed is proposed. However, in the light collecting system, all heliostats are made to belong to and collect light to a nearest tower. Therefore, when the light collecting system is installed in the northern hemisphere for instance, the number density of the heliostats is large on a south side of the tower causes the light collecting system to be inefficient (, that is, the number of heliostats required to obtain the same quantity of light increases).
By the way, in conventionally experimented or proposed sunlight collecting systems, light is interfered between a remote heliostat placed far from the tower and its adjacent heliostats. To prevent this, the heliostats need to be sparsely distributed. In other words, a lot of heliostats are placed on the ground with proper intervals between each other so as to avoid the light interference. However, in a position far from the tower, long intervals are required to avoid the interference of the reflected light (referred to as “blocking”, hereafter) between the adjacent heliostats. Accordingly, the reflectors need to be sparsely distributed. As a result, there are the following problems (a) and (b).
(a) There are unused sunny ground irradiated by the sunlight, in which the heliostats to reflect the sunlight cannot be placed. Therefore, only a part of the sunlight which irradiates the ground is used. For instance, in the conventional light collecting system, the efficiency of sunlight utilization at noon on an equinox in the latitude of Japan is estimated at about 40%. Moreover, the efficiency decreases as the light collecting system enlarges to increase a quantity of collected light.
(b) Lengths of light paths of the reflected light from the heliostats to the heat collector or the light collecting reflector become long. Therefore, a focus on a reflecting surface (a focal plane) becomes relatively wide. As a result, the following problems also occur.
(b-1) The heat collector or the light collecting reflector needs to be so large that the light collecting power decreases.
(b-2) Since the light collecting power decreases, a temperature of heat exchange medium in the heat collector which collects thermal energy of the collected sunlight decreases. This remarkably influences a beam-down light collecting system shown in FIG. 33.
Such a problem that the efficiencies of the sunlight collection and utilization decrease as the light collecting system enlarges is more prominent in an enlarged light collecting system where the heliostats are distributed in a wider area, and a bottleneck to construct a large-scale light collecting system.
Non-Patent Document 1: J. E. Pacheco and R. Gilbert, “Overview of Recent Results for the Solar Two Test and Evaluations Program.” Proceedings of the 1999 ASME International Solar Energy Conference: Renewable and Advanced Energy Systems for 21st Century, Maui, Hi. (1999).
Non-Patent Document 2: E. Epstein, A. Segal and A. Yogev, “A molten salt system with a ground base-integrated solar receiver storage tank.” J. Phys. IV France 9, 95-104 (1999).
Non-Patent Document 3: Phillipp Schramek, David R. Mills, “Multi-tower solar array”, Solar Energy 75 (2003) 249-260.